riscv: Use PMD_SIZE to replace PTE_PARENT_SIZE

The PMD_SIZE is equal to PGDIR_SIZE when __PAGETABLE_PMD_FOLDED is
defined.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
[paul.walmsley@sifive.com: fixed spelling in commit summary]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>